This invention relates to improved electrodes for a membrane electrode assembly (MEA) for use in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC), and more particularly a method of manufacturing nanotubes formed from platinum (Pt) and platinum alloys, for example, platinum-palladium (PtPb). The present invention includes, but is not limited to, improved cathodic catalysts formed from such platinum nanotubes (PtNTs) and platinum-palladium nanotubes (PtPdNTs).
A fuel cell is a device that converts the chemical energy of a fuel and an oxidant directly into electricity without combustion. The principal components of a fuel cell include electrodes catalytically activated for the fuel (anode) and the oxidant (cathode), and an electrolyte to conduct ions between the two electrodes, thereby producing electricity. The fuel typically is hydrogen or methanol, and the oxidant typically is oxygen or air.
Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy. Compared with internal combustion engines, fuel cells are not limited by the Carnot cycle and in principle could have higher efficiency. With pure hydrogen as the fuel, fuel cells are very environmentally friendly. The combination of high efficiency, low environmental impact, and high power density has been and will continue to be the driving force for vigorous research in this area for a wide variety of applications such as transportation, residential power generation, and portable electronic applications. For portable electronic applications, important features include high power density (i.e., longer battery life) and compactness.
Silicon-based microfabrication technology is amongst the promising approaches for fabrication of compact micro fuel cells. However, the current methods for making electrodes for fuel cells, which typically includes spraying and/or brushing of platinum supported on carbon powder, is incompatible with microfabrication techniques. Therefore, there is need for improved electrodes and methods of preparing such electrodes for PEMFCs and DMFCs.
Direct methanol fuel cells (DMFCs) have attracted enormous attention as a promising power source for portable electronics applications such as laptop computers and cell phones. The interest in commercializing DMFCs is in part due to the fuel cell's simple system design, high energy density and the relative ease with which methanol may be transported and stored, as compared with hydrogen. In the state-of-the-art DMFCs, platinum supported on a carbon substrate is configured in the cathode as a catalyst for activating the oxygen reduction reaction (ORR). A platinum-ruthenium alloy is usually used as the anode electrocatalyst, and may be supported on a carbon substrate. The electrolyte is usually a perfluorosulfonate membrane, for which NAFION (available from DuPont) is a commonly utilized commercially available membrane. One of the major problems encountered in DMFCs is methanol crossover from the anode to the cathode. The permeated methanol causes “poisoning” of the cathode platinum catalyst and depolarization losses due to the simultaneous oxygen reduction and methanol oxidation on the platinum catalyst.
Reference is made herein to the well-known rotating disk electrode (FIG. 28), which is used in the testing of the present invention as described below. As will be appreciated by those of ordinary skill in the art, the rotating disk electrode (RDE) consists of a disk on the end of an insulated shaft that is rotated at a controlled angular velocity. Providing the flow is laminar over all of the disk, the mathematical description of the flow is surprisingly simple, with the solution velocity towards the disk being a function of the distance from the surface, but independent of the radial position. The rotating disk electrode is used for studying electrochemical kinetics under conditions, such as those of testing the present invention, when the electrochemical electron transfer process is a limiting step rather than the diffusion process.
Polymer electrolyte based low temperature fuel cells, with their two best known variants, proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC), have been considered promising for powering automobiles, homes, and portable electronics. Their successful commercialization is, however, very much dependent on the activity and durability of their electrocatalysts. At present, all pre-commercial low temperature fuel cells use supported Pt and Pt alloys as their electrocatalysts. The critical properties to consider when choosing an electrocatalyst support include its electrical conductivity, surface area, macro-morphology, microstructure, corrosion resistance, and cost. Carbon black (CB), such as Vulcan XC-72, has been the most widely used electrocatalyst support because of its reasonable balance among electronic conductivity, surface area, and cost. Recently, many nanostructured carbon materials with graphitic structure, such as nanotubes (CNTs), nanofibers (CNF) nanocoils, nanoarrays and nanoporous hollow spheres, have been studied. Among them, CNTs are of particular interest due to their unique electronic and micro and macro structural characteristics. CNTs have also been shown to be more corrosion-resistant than CB under simulated fuel cell operation conditions.
Among the two variants of low temperature fuel cells, DMFCs have been attracting great attention for powering small devices, such as laptop computers, cell phones, and personal digital assistants, because of their high energy density, ease of handling liquid fuel, and low operating temperature. However, the slow electrokinetics of the anode reaction—a methanol oxidation reaction—is still a key problem to the commercialization of DMFCs. Normally, expensive noble metal alloys, typically Pt—Ru, with a high electrode metal loading (e.g., >2.0 mg/cm2) are employed in order to offer a reasonable fuel cell performance (e.g., 80 mW/cm2 at cell temperature of 90° C. and O2 pressure of two atmospheres). It has long been desired for a high performance anode catalyst to be developed so that the electrode metal loading and thus the cost of DMFCs can be reduced.
Some early investigations have found that, by simply replacing CB with CNTs in the conventional ink-paste electrode fabrication method, superior DMFC performance can be obtained. For example, a DMFC single cell with cup-stacked CNTs supported Pt—Ru anode catalyst showed nearly three times the maximum power density of a DMFC with CB (Vulcan XC-72) supported Pt—Ru anode catalyst, and it was suggested that CNTs can provide better charge and mass transfer.
Several types of carbon nanotubes may be used as electrocatalyic supports for low temperature fuel cells, for example, single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and double-walled carbon nanotubes (DWNTs). SWNTs can have a large surface area (e.g., 500-1000 m2/g) due to their small diameter (e.g., one nm), which is a favorable property as catalysts support. However, they normally contain a significant amount (e.g., two-thirds) of semiconducting tubes, which are poor electron conductors and thus are expected to be a poor electrocatalytic support. MWNTs are highly conducting, but they have limited surface area (e.g., 100-200 m2/g) due to their large diameter (e.g., forty nm). It was recently shown that most DWNTs are conducting tubes and that they can have high surface areas (e.g., 500-1000 m2/g). Thus a natural and logical choice for an electrocatalyst support is DWNTs.
The slow rate of the oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cell (PEMFC) is the main limitation for automotive applications. It has been shown that the Pt3Ni(111) is 10-fold more active for the ORR than the corresponding Pt(111) surface and 90-fold more active than the current state-of-the-art Pt/C catalysts for PEMFC. The Pt3Ni(111) surface has an unusual electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region. Under operating conditions relevant to fuel cells, its near-surface layer exhibits a highly structured compositional oscillation in the outermost and third layers are Pt rich and the second atomic layer is Ni rich. The weak interaction between the Pt surface atoms and non-reactive oxygenated species increases the number of active sites for O2 adsorption.
The utilization of the PEM fuel cell in demanding application such as automobiles must be overcome the kinetic limitations on the oxygen reduction reaction (ORR), which have led to three fundamental problems. First, the significant overpotential for the oxygen reduction reaction, at practical operating current densities reduces the thermal efficiency well below the thermodynamic limits, typically about 43% at 0.7 V versus the theoretical thermal efficiency of 83% at the reversible potential for the ORR (1.23 V). Second, an approximately five-fold reduction of the amount of Pt (platinum-loading) in current PEMFC stacks is needed to meet cost requirements for large scale automotive applications. Finally, the dissolution and/or loss of Pt surface area in the cathode must be greatly reduced.
These limitations could be eliminated if stable cathode catalysts with an order of magnitude increase in the specific activity over state-of-the-art Pt/C catalysts can be developed. In the hope that a combination of different metals would have improved catalytic activity and stability relative to a pure metal, the ORR has been studied on numerous bi (or multi) metallic alloys. These studies have led to incremental improvements to catalyst performance, but large increases in activity have yet to be realized.
Considering that the Pt3Ni(111)-skin surface exhibits the highest catalytic activity that has ever been detected, the challenge would be to create nanocatalyst with electronic and morphological properties that mimic the Pt3Ni(111) surface. In the future, therefore, a way to reduce the current specific power density in fuel cell (0.7 gPt/kW) without a loss in cell voltage, while maintaining the maximum power density (W/cm2), would be engineering of PtNi(111)-skin like nanocatalysts.
Fuel cells, as devices for direct conversion of the chemical energy of a fuel into electricity by electrochemical reactions, are among the key enabling technologies for the transition to a hydrogen-based economy. Of several different types of fuel cells under development today, polymer electrolyte fuel cells (PEFCs) have been recognized as a potential future power source for zero-emission vehicles. However, to become commercially viable, PEFCs have to overcome the barrier of high catalyst cost caused by the exclusive use of platinum and platinum-based catalysts in the fuel-cell electrodes. Here we demonstrate a new class of low-cost (non-precious metal)/(heteroatomic polymer) nanocomposite catalysts for the PEFC cathode, capable of combining high oxygen-reduction activity with good performance durability.
A schematic representation of the principle of fuel-cell operation is shown in FIGS. 1 and 2. PEFCs operate with a polymer electrolyte membrane that separates the fuel (hydrogen) from the oxidant (air or oxygen). Precious-metal catalysts, predominantly platinum (Pt) supported on carbon, are used for both the oxidation of the fuel and reduction of the oxygen in a typical temperature range of 80-100° C. Apart from the issue of the high cost of catalyst and other fuel-cell system components (polymer electrolyte membrane, bipolar plates, the rest of the power system, and so on), PEFCs suffer from insufficient performance durability, arising mainly from cathode catalyst oxidation, catalyst migration, loss of electrode active surface area, and corrosion of the carbon support. In a direct methanol fuel cell (DMFC), the Pt cathode also endures a performance loss resulting from the lack of tolerance to methanol diffusing through the membrane from the anode side of the cell. Thus, whether using hydrogen or methanol as a fuel, PEFCs are in need of efficient, durable and, most importantly, inexpensive catalysts, as alternatives to Pt and Pt-based materials. Although ideally the Pt catalyst should be replaced at both fuel-cell electrodes, the substitution of the cathode catalyst with a non-precious material is likely to result in significantly greater reduction of Pt needed for PEFCs. This is because the slow oxygen reduction reaction (ORR) at the cathode requires much more Pt catalyst than the very fast hydrogen oxidation at the anode.
Two approaches are at present gaining momentum to replace Pt, which is scarce (only 37 p.p.b. in the Earth's crust) and expensive (˜US$45 g−1, the highest Pt price in 25 years). One approach uses non-Pt catalysts that nonetheless contain precious metals with limited abundance and/or limited world distribution. Such catalysts are typically based on palladium (Pd) or ruthenium (Ru). Although Pt is thus avoided, the result is the replacement of one precious metal with another that is on the whole less active than Pt. An alternative approach is to replace Pt with abundant, non-precious materials that are not susceptible to price inflation under high-demand circumstances. There has been demonstrated a new non-precious composite catalyst from an entirely new class of (non-precious metal)/(heterocyclic polymer) composite materials, synthesized via a pyrolysis-free process. As shown with the Co—PPY—C composite, such catalysts promise to deliver high ORR activity without any noticeable loss in performance over long fuel-cell operating times. This therefore opens up the possibility of making a variety of other non-precious composite materials from this class of composites for use as catalysts for the PEFC cathode.
Fuel cells are expected to become a major source of clean energy with particularly important applications in transportation. Despite considerable recent advances, existing fuel-cell technology still has drawbacks, including the instability of the platinum electrocatalyst for the ORR at the cathode. Recent work recorded a substantial loss of the Pt surface area over time in proton-exchange membrane fuel cells (PEMFCs) during the stop-and-go driving of an electric car; this depletion exceeded the Pt dissolution rates observed upon holding at constant potentials for extended time spans. The reported study results suggest possibilities toward resolving this impediment and for synthesizing improved ORR Pt-based and for stabilizing Pt and other Pt-group metals under oxidizing conditions.
Accordingly, there is a need for, and what was heretofore unavailable, an improved membrane electrode assembly incorporating electrocatalysts formed from platinum and platinum based alloy materials having increased durability and decreased costs. The present invention satisfies these and other needs